Comparison of aerosol optical thickness measurements by MODIS, AERONET sun photometers, and Forest Service handheld sun photometers in southern Africa during the SAFARI 2000 campaign

The Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on the NASA Terra satellite has been used to monitor aerosol optical thickness (AOT, tau) daily at 10 km x 10 km resolution worldwide since August 2000. This information, together with the locations of active fires detected by the MODIS instrument, is essential for understanding the seasonal trends and interannual variability of fires and their impacts on air pollution, atmospheric chemistry, and global climate. We compared aerosol optical thickness derived from MODIS, five automated sun photometers of the Aerosol Robotic Network (AERONET), and 38 Forest Service (FS) handheld sun photometers in western Zambia from 20 August to 20 September 2000. Aerosol optical thicknesses derived from AERONET sun photometers and FS sun photometers were also compared in the same region between mid-June and late September 2000. Our objectives were to validate the AOT measurements by MODIS and to investigate the factors that affect AOT measurements. We demonstrated that in the regions of intense biomass burning, MODIS aerosol optical thickness was consistently 40-50% lower at 470, 550, and 660 nm compared with ground-based AOT measurements by automated and handheld sun photometers and airborne measurements by NASA Ames Airborne Tracking 14-channel Sunphotometers (AATS-14). The satellite look angles can influence the MODIS AOT values, with the actual MODIS AOT values being as much as 0.06 higher than model-calculated MODIS AOT values on the right edge of the MODIS scene. This phenomenon may be due to error in the assumed aerosol scattering phase function or surface directional properties. Density of vegetation cover can also affect MODIS measurements of aerosol optical thickness.